In solid ink imaging systems having intermediate members, ink is loaded into the system in a solid form, either as pellets or as ink sticks, and transported through a feed chute by a feed mechanism for delivery to a heater assembly. A heater plate in the heater assembly melts the solid ink impinging on the plate into a liquid that is delivered to a print head for jetting onto an intermediate transfer member which may be in the form of a rotating drum, for example. In the print head, the liquid ink is typically maintained at a temperature that enables the ink to be ejected by the printing elements in the print head, but that preserves sufficient tackiness for the ink to adhere to the intermediate transfer drum. In some cases, however, the tackiness of the liquid ink may cause a portion of the ink to remain on the drum after the image is transferred onto the media sheet which may later degrade other images formed on the drum.
To address the accumulation of ink on a imaging drum, solid ink imaging systems may be provided with a drum maintenance unit (DMU). In solid ink imaging systems, the DMU is configured to 1) lubricate the image receiving surface of the drum with a very thin, uniform layer of release agent (e.g., silicone oil) before each print cycle, and 2) remove and store any excess oil, ink and debris from the surface of the drum after each print cycle. Previously known DMU's typically included a reservoir for holding a suitable release agent, an applicator that receives oil from the reservoir and applies the oil to the surface of the drum, and a metering blade for metering the oil applied to the surface of the drum by the applicator.
DMU's have an expected lifetime, or useful life, that corresponds to the amount of oil stored in the reservoir, often correlated to a number of prints that the DMU is capable of providing adequate oil for image transfer. For example, some DMU's may have a useful life between approximately 300,000 and 500,000 prints depending on factors such as oil usage and the amount of oil in the reservoir. One factor that affects the useful life of a DMU is excess oil being delivered to the drum over time.
Excess oil being delivered to drum, in turn, may be caused by metering blade wear. Metering blade wear is, in part, determined by the arrangement of the metering blade with respect to the drum, also referred to as the mode of the metering blade. In previously known systems, the metering blade was arranged in either a “wiper mode” or a “doctor mode.” In wiper mode, the metering blade is arranged with the blade tip oriented to wipe or squeegee the oil on the surface of the drum, and, in doctor mode, the metering blade is arranged with the blade tip oriented against the direction of rotation of the drum, similar to a chisel. Blade wear is typically faster when in the wiper mode due to the pressure that is exerted on the blade to keep the blade tip adjacent the drum surface for metering the oil. Conversely, blade wear is less when the metering blade is arranged in the doctor mode.
A disadvantage of the use of the metering blade in doctor mode is the formation of an oil bar on the drum surface. In previously known metering blade configurations, the metering blade had a square tip. In doctor mode, the square-end of the blade acts as a dam and traps a large bead of oil on the image drum surface. During operation, the metering blade is typically moved into and out of engagement with the drum. When the blade is disengaged from the drum, the bead of oil splits leaving an oil bar on the surface of the drum. Depending on the size of the oil bar, an amount of oil may be left on the drum after the metering blade is disengaged from the drum that unnecessarily increases the amount of oil that is used per print, thus decreasing the useful life of the DMU.
In addition to decreasing the useful life of a DMU, excess oil on the drum surface, due to metering blade wear or oil bar size, may result in an image quality defect known as “duplex dropout.” In sustained duplex printing, oil applied by the DMU to the drum is transferred to the “front” side of the paper while printing the front side of the paper and then from the “front” side of the paper to the transfix roll during the printing of the back side (i.e., duplex side) of the paper. During subsequent printing, oil that gets on the transfix roll may be transferred to the “back” side of a sheet of paper when printing on the front side. When excess oil is delivered to the drum during the front side print step, the thickness or amount of oil on the front side of the paper may interfere with the transfer of the image to the back side of the paper resulting in some or all of the image not transferring to the back side of the sheet, also referred to as “duplex dropout.”